1. Technical Field
The present invention relates to a dielectric filter comprising a plurality of resonators and it also relates to a method of regulating the frequency bandwidth of such a dielectric filter.
2. Related Art
There have been proposed a variety of high frequency dielectric filters each comprising a dielectric ceramic block having a rectangularly parallelepipedic shape in which three or more through bores are formed in parallel with each other, the inner surface of each through bore is covered with an internal conductive layer or film operating as a resonant conductor, one of the oppositely disposed end surfaces of the dielectric ceramic block at which openings of the through bores are positioned is devoid of a shield electrode to form an electrically open-circuit end surface, the other end surface is provided with a shield electrode to form a short-circuit end surface along with the remaining four lateral side surfaces of the dielectric ceramic block. With a dielectric filter having such an arrangement, each of the resonant conductors operates as a resonator.
Referring to FIG. 1 of the accompanying drawings illustrating an equivalent circuit diagram of a dielectric filter of the type under consideration comprising three resonators Q1, Q2 and Q3, the filter can be polarized by coupling the resonators Q1 and Q2, and Q2 and Q3 respectively by coupling capacitances C1 and C2 to produce an attenuation pole on the lower frequency side of the center frequency. In FIG. 1, reference symbols L1 and L2 respectively denote inductances representing inductive couplings between the resonators Q1 and Q2, and Q2 and Q3, respectively.
In such a dielectric filter, chip capacitors are conventionally used for realizing or providing coupling capacitances C1 and C2 for the purpose of polarization and connected in series with the resonators.
However, such an arrangement requires a large number of components, which by turn make the assembling operation cumbersome and the assembled filter rather bulky.
In order to bypass this problem, as illustrated in FIG. 2 there has been proposed a dielectric filter comprising a rectangularly parallelepipedic dielectric ceramic block A in which three resonators made of resonant conductors B1, B2 and B3 that are arranged in parallel with each other along a given direction. Additionally, the first and second resonant conductors B1 and B2 are commonly flanked by a first capacitive electrode D1 at a position close to the electrically open-circuit end surface while the second and third resonant conductors B2 and B3 are similarly and commonly flanked by a second capacitive electrode D2 in such a way that the capacitive electrodes D1 and D2 are electrically insulated from shield electrodes E provided on the lateral side surfaces of the dielectric ceramic block A so that electrostatic capacitances are respectively produced by the capacitive electrodes.
Meanwhile, there has been an increasing demand for dielectric filters each having a large effective bandwidth, although those described above do not and cannot meet the demand unless a method of regulating the frequency bandwidth is provided which affords a desired wide bandwidth.
It is, therefore, one object of the present invention to provide a dielectric filter having a configuration as described above that shows a large effective bandwidth.
Another object of the present invention is to provide a method of regulating the frequency bandwidth of a dielectric filter in order to achieve such a large effective bandwidth.